1. Field of the Invention
The present invention generally relates to semiconductor integrated circuits, and particularly relates to a semiconductor integrated circuit which operates based on an oscillating signal generated by an oscillator.
2. Description of the Related Art
In many types of semiconductor integrated circuits, operations are controlled based on a clock signal that is generated by an oscillator. In order to generate a highly precise clock signal, a crystal oscillator is typically used. A crystal oscillator oscillates with highly precise oscillating frequency by utilizing a resonance effect of an external quartz resonator. Such crystal oscillator is typically 1000 times or more accurate than RC oscillator circuits, and also exhibits low temperature dependency of frequency.
Immediately after a semiconductor integrated circuit is powered on, the oscillating signal is not yet stable because the oscillation of the oscillator has just begun. The amplitude of the oscillating signal is not sufficient during this waiting period before the stabilization of oscillation, and the oscillation waveform may easily be disturbed. If the oscillating signal in such a state is used as a clock signal in the semiconductor integrated circuit, the circuit may be running a risk of suffering malfunction.
In order to avoid such circuit malfunction, provision may be made to start the operation of the semiconductor integrated circuit after waiting for the oscillating signal of the oscillator to stabilize. Conventional technologies for waiting for the oscillating signal of an oscillator to stabilize include a semiconductor integrated circuit in which a timer and a stabilization-wait-setting register are provided, with the output of the timer being compared with the content of the stabilization-wait-setting register. In this configuration, a check is made to determine whether the time lapse indicated by the output of the timer exceeds the time setting stored in the stabilization-wait-setting register. If the time lapse exceeds the time setting, operation is allowed to start by resetting an internal reset of the semiconductor integrated circuit based on the premise that a sufficient time lapse is observed.
Further, Japanese Patent Application Publication No. 2003-216265 discloses a configuration that detects the stabilization of oscillation by use of an oscillation amplitude detecting means. This oscillation amplitude detecting means detects that the oscillation amplitude exceeds a predetermined level by utilizing a threshold of a buffer circuit.
In oscillation circuits, the oscillation stabilization time, which is a time length required for the stabilization of oscillation, differs depending on individual products and operation environments due to differences in resonator characteristics and operation conditions. The above-noted configuration that compares the output of a timer with the content of a stabilization-wait-setting register does not take into account such variations depending on individual products or operation environments, and sets aside a fixed time length as the time length required for the stabilization of the amplitude. Accordingly, a small margin, if used, cannot guarantee reliable operation, and a large margin, if used for the purpose of securing sufficient reliability, results in a needless wait time being increased.
Accordingly, there is a need for a semiconductor integrated circuit which can reliably detect the state of stable oscillation without being affected by a shortening/elongating of the oscillation stabilization time due to differences in individual products and operation environments.